We read with interest the article by Lackner et al.1 in which an optotype contrast sensitivity test was used to measure contrast thresholds before and after laser in situ keratomileusis. The experiment is conceptually interesting; however, we have serious concerns about the methodology.
The authors wished to test only patients in whom the pupil diameter was smaller than the ablation zone under “mesopic conditions,” which are not defined. We are not told whether these mesopic conditions were actually equivalent to the contrast threshold test conditions, only that testing was performed in a “darkened room.” Patients with negative clearance were excluded based on preoperative pupil measurements performed with continuous infrared photoretinoscopy using equipment designed to dynamically test accommodative miosis. In an earlier article,2 the authors used the same equipment to measure 10 patients with a mean age of 38 years (range 30 to 55 years) and found a mean “dark-adapted” pupil diameter of 5.35 mm (range 4.82 to 5.79 mm). This value is so small that it probably does not represent an accurate measurement of fully dark-adapted pupils.3 Not a single pupil was 6.0 mm or larger, despite the inclusion of patients 30 to 35 years old.4 In the current article, the mean age was 33 years (range 21 to 41 years) and the mean pupil diameter was 6.2 mm (range 5.0 to 7.8 mm), which is still on the low side. Although the ablation zone was 5.5 or 6.0 mm, the authors state, “In all 15 cases, the ablation zone exceeded the dark-adapted pupil size.” The internal contradictions of the current report make it uninterpretable.
Also, the authors performed optotype contrast threshold testing using the AcuityMax software platform (www.Science2020.com). We could find no information about the design of the test, Early Treatment Diabetic Retinopathy Study compliance, or the availability of experimentally determined age-normal contrast threshold values.5
Finally, we disagree with the authors' concept that “[a]n appropriate evaluation of optical properties after refractive surgery therefore requires a pupil size smaller than the ablation zone.” Manipulating the pupil diameter experimentally to eliminate negative clearance is a perversion of the intent of such research. An appropriate evaluation of the functional optical quality of the ablation profile requires a standardized protocol for room illumination and pupil adaptation. The pupils may then do what they will, and the patients will tell us what they see.
Sandra M. Brown MD
Arshad M. Khanani MD
Lubbock, Texas, USA
1. Lackner B, Pieh S, Funovics MA, et al. Influence of spectacle-related changes in retinal image size on contrast sensitivity function after laser in situ keratomileusis. J Cataract Refract Surg 2004; 30:626-632
2. Lackner B, Pieh S, Schmidinger G, et al. Glare and halo phenomena after laser in situ keratomileusis. J Cataract Refract Surg 2003; 29:444-450
3. Brown SM, Khanani AM, Xu KT. Day to day variability of the dark-adapted pupil diameter. J Cataract Refract Surg 2004; 30:639-644
4. Loewenfeld IE. Pupillary changes related to age. In: Thompson HS, Daroff R, Frisèn, et al, eds, Topics in Neuro-Ophthalmology. Baltimore, MD, Williams & Wilkins, 1979
5. Khanani AM, Brown SM, Xu KT. Normal values for a clinical test of letter-recognition contrast thresholds. J Cataract Refract Surg 2004; 30:2377-2382